Hydraulic actuators utilize a fluid for the movement of an output element, and these types of actuators are used in a variety of applications including numerous mechanical and aeronautical applications. Hydraulic actuators may be used with hydraulic flow control valves in servomechanisms to control machines and structures, and are sometimes referred to as servoactuators. Servoactuators are commonly used on rotary and fixed-wing aircraft to counter and control the large forces that develop on the flight control surfaces experience during operation of the aircraft. For example, servoactuators may be used in rotary aircraft to position equipment such as a swashplate. Servoactuators may also be used in fixed wing aircraft to position equipment such as nose wheel landing gear, main landing gear, speed brake control surfaces, flap control surfaces and primary flight control surfaces.
For some applications, such as those above and others, it may be desirable to use tandem hydraulic pistons, i.e., pistons that have two or more heads. For tandem pistons, redundant flow control valves and hydraulic systems may be used so that the hydraulic system, e.g., a flight control system, is able to function when one of the hydraulic systems fails. When two piston heads are present, the actuator may be referred to as a tandem or dual-tandem actuator or servoactuator.
In typical redundant dual-tandem servoactuators, two mechanical flow control valves are required to provide necessary flow control redundancy. Redundancy may include the ability to provide flow control to an actuator in the event of a hydraulic supply failure or a control valve jam, i.e., a situation where the control valve spool jams or seizes within the sleeve of the control valve. For example, in aircraft applications, it is desirable for a redundant dual-tandem servoactuator to have a fail-operate capability that allows the pilot to mechanically operate the servoactuator with one hydraulic system following a single hydraulic supply failure in the other hydraulic system. It is also desirable for a redundant dual-tandem servoactuator to have a fail-operate capability which allows the pilot to mechanically operate both flow control valves following a jam or seizure in one of the flow control valves.
A variety of redundant servoactuators have been used for such safety purposes, however these have often utilized the relatively high system pressures of the hydraulic supply. Such pressures are typically measured in thousands of pounds per square inch (ksi). Such high pressures can create large bias forces in the chambers of a non-operational piston/cylinder assembly after a single hydraulic system failure, which in turn can make movement of a failed system difficult for an operator and thus may negate advantages of having a redundant actuator system.
For the foregoing reasons, there is a need for redundant flow control for hydraulic actuators that provides fail-operate capability with subsequent low pressure and low bias forces in a failed actuator system.